A mate of mine has a bunch of TA quals (Para trained etc.) but is looking at an FTRS with a non-airborne regular unit. Would his pay be reduced to a standard Private soldier or would he keep para pay (or Cdo/SF pay) for the duration of his FTRS?

But that is the point - a Para Regt soldier is always Para Regt unless he transfers to another cap badge. A para trained gunner (for example) is a gunner with an additional qualification which for pay purposes is only 'live' while in an airborne PID. That was the crux of the para pay debate - limiting para pay to attached arms only while in role/ PID.

The Para Regt bit is a distraction anyway as I believe the OP is asking about non Para Regt going to a non-para PID, so no pay.

Parachute Regt (up to and including Lt Col) receive SP (Para) on a Continuous Career Basis. All others, including RN, RM, and RAF receive SP (Para) on an Non-Continuous Basis.

CCB SP is paid where the specialism is fundamental to the core role of the individual, and will remain so for the duration of their career providing they remain qualified. NCB SP is paid where the specialism is a secondary skill for the individual, but is a core task within the unit in which the qualifying post has been established. Individuals move in and out of the unit/post in question and, providing they are qualified, whilst in a qualifying post they receive SP.